* contrib/lisp/org-contacts.el (org-contacts-filter): Add a PROP-MATCH
argument.
(org-contacts-gnus-article-from-get-marker): Search for email as a
property not a tag.
`org-contacts-gnus-article-from-get-marker' seems to assume that the
contact properties exist within the contact's tags as colon-separated
KEY={VALUE} entries. This isn't the case for me, using org-mode from
git master.
This commit adds a PROP-MATCH argument to `org-contacts-filter' so
that `org-contacts-gnus-article-from-get-marker` can filter by the
EMAIL property.